Add pinned JuliaFormatter tooling and CI format check

[garrison]

What

Adds a pinned formatting setup so contributors and CI all format with the exact same JuliaFormatter version, regardless of what's installed globally.

Changes

• format/ — a self-contained environment that pins JuliaFormatter to 2.8.5 (Project.toml [compat] = "=2.8.5" + a committed Manifest.toml). Mirrors the existing gen/ pattern.
• Makefile — short task runner:
  • make format — format the repo with the pinned formatter
  • make format-check — fail (non-zero exit) if anything is unformatted; used by CI
  • make test, make gen — convenience wrappers for the existing commands
• .github/workflows/CI.yml — new Format check job (Linux-only, since formatting is platform/version independent) running make format-check on PRs and the merge queue.
• AGENTS.md — documents the format/ env and the make targets.

Why

The old instruction julia -e 'using JuliaFormatter; format(".")' relied on whatever JuliaFormatter happened to be in the user's global environment, giving inconsistent results across contributors and CI. Pinning in a dedicated env removes that drift without polluting anyone's global environment.

Notes / follow-up

• To make this a hard merge gate, "Format check" still needs to be added to the required status checks in branch protection / merge-queue settings for main.
• To bump the version later: edit format/Project.toml [compat], run julia --project=format -e 'import Pkg; Pkg.update()', and commit the updated format/Manifest.toml.

🤖 Generated with Claude Code

[coveralls]

coverage: 76.4%. remained the same — add-pinned-formatter-tooling into main